Defective DNA synthesis in human megaloblastic bone marrow: effects of homocysteine and methionine

Abstract

A B S T R A C T In Bin deficiency, inadequate DNA synthesis seems due in large measure to a block of tetrahydrofolic acid (THFA) regeneration from 5-methyl THFA (via homocysteine transmethylation).In support of the above, homocysteine appears to facilitate and methionine to reduce de novo DNA synthesis. This was measured by the ability of deoxyuridine to suppress thymidine-8H uptake into DNA in human bone marrow cultures. The homocysteine effect in B,2-deficient marrow supports the possibility that there is in man an additional Bin-independent pathway for regeneration of THFA by methylation of homocysteine to form methionine.Among possible explanations for the methionine effect is end-product inhibition of the homocysteine transmethylase reaction, resulting in further accumulation of 5-methyl THFA. Homocysteine transmethylation may play an important role in the regulation of THFA availability and de novo DNA synthesis.In vitro and in vivo evidence suggests that methionine may be useful to potentiate and homocysteine to reduce methotrexate action.

The effect of homocysteine, methionine, serine and glycine on DNA synthesis by human normoblastic and megaloblastic bone marrow cells.

“…Theoretically however, the addition of Met should not effect this pathway in megaloblastic marrow cells because it was already suppressed. But the report of WAxMAN et al (4) showing no effect of Met on DUS test in vitamin B12-deficient marrow was not confirmed by the present study and by that of CHENG et al (5). No difference was observed in the effect of Met on the normoblastic and megaloblastic marrow.…”

“…In addition, Met alleviates the effects of vitamin B12 deficiency in rats and this aspect has been reviewed by SHIN et al (3). The effect of homocysteine (Homo) and Met on the DUS test has been reported by WAXMAN et al (4) and CHENG et al (5).…”

Biochemical mechanisms in the Killmann experiment: critique of the deoxyuridine suppression test.

“…This hypothesis has occasioned much debate (7)(8)(9)(10)(11), some of it centering on the merits of an experiment by Killmann (12), which later workers made the basis of the deoxyuridine suppression test (13)(14)(15)(16)(17). With ra-dioautographic methods Killmann showed that exogenous deoxyuridine strongly inhibits (or suppresses) the uptake of labeled thymidine into normal bone marrow cells, but does so by a smaller percentage in bone marrow cells from cobalamin-deficient subjects.…”

Mild Methionine Excess Does Not Affect Thymidylate Synthesis or Inflammation Markers Expression in Human Aortic Endothelial Cells

“…The inhibition of the de novo thymidylate synthesis by excess methionine was first described in bone marrow cells [4,5] and was also reported in Raji cells, a Burkitt lymphoma cell line [7] . In this latter study, it was suggested that the most likely mechanism of inhibition involved the metabolite homocysteine, and not unmetabolized L -methionine.…”

“…Classical studies have described depressions in growth and food intake, as well as lesions of the liver, spleen, kidney, pancreas and small intestine when excessive doses are given to experimental animals [3] . At the cellular level, methionine has been shown to lower the de novo synthesis of thymidylate and thymine-DNA in bone marrow cells [4][5][6] . It has also been reported that high concentrations of L -methionine inhibit the de novo synthesis of thymidilic acid in Raji cells, a Burkitt lymphoma cell line [7] .…”